1. Field of the Invention
The present invention relates to a squeezing device of a wet printer and a developing unit employing the same, and more particularly to a squeezing device of a wet printer and a developing unit, capable of stabilizing image forming conditions for a photosensitive medium. The present application is based on Korean Patent Application No. 2001-8777, which is incorporated herein by reference.
2. Description of the Related Art
FIG. 1 is a view for schematically showing a general wet printer.
As shown in FIG. 1, a wet printer is equipped with a photosensitive belt 1, light scanning units 4, a transfer unit 7, and a plurality of developing units 10.
The light scanning units 4 each scan light to the photosensitive belt 1 charged with a certain potential to correspond to image information. The developing units 10 are provided for the respective colors, supply ink to the photosensitive belt 1, and develop an electrostatic latent image that is formed on the photosensitive belt 1 by the light scanning units 4. The transfer unit 7 transfers a toner image formed on the photosensitive belt 1 to supplied sheets of paper.
The developing units 10 include a container 16 for reserving ink mixed with liquid carrier element and toner in a predetermined ratio, developing rollers 12, ink suppliers 17 for supplying ink of certain colors reserved in the containers 16 between the photosensitive belt 1 and the developing rollers 12, setup rollers 13, and squeezing rollers 14.
The developing rollers are each installed to form a nip which is a minute gap from the photosensitive belt 1, and biased with a certain potential to fix supplied ink on an electrostatic latent image region of the photosensitive belt 1 by an electric force. The setup rollers 13 are installed to form a minute gap from the photosensitive belt 1 in order to separate ink oversupplied to the photosensitive belt 1 from the photosensitive belt 1 by the surface tension.
The squeezing rollers 14 separate the liquid carrier element not participating in the formation of an image from the photosensitive belt 1 and collect the separated liquid carrier element into a collecting reservoir 11.
A reference numeral 15 indicates backup rollers each of which restrains the looseness of the photosensitive belt 1 and supports the squeezing roller 14 to stably contact the photosensitive belt 1.
As shown in FIG. 2, between the squeezing rollers 14 and the photosensitive belt 1 are defined squeeze nips that are contact regions for the squeezing roller 14 and the photosensitive belt 1. Substantially, these squeeze nips may be minute gaps between the squeezing rollers 14 and the photosensitive belt 1 which are positioned side by side at a certain distance from each other.
The squeezing rollers 14 include a core 14a of a conductive material and a contact layer 14b of a rubber material surrounding the outer circumference of the core 14a. Further, the squeezing rollers 14 can apply a bias potential to the core 14a in order to build a condition in which another electrostatic latent image of another color is to be formed on the photosensitive belt 1.
Ink developed on the photosensitive belt 1 has conductivity, but the liquid carrier included in the ink is a dielectric substance preventing electricity from flowing. However, if an electric field stronger than a critical potential is applied to the squeeze nips, a townsend discharge occurs so that electric currents flow in the carrier. Accordingly, in order to charge the photosensitive belt 1, a bias voltage over the critical potential should be applied to the core 14a of the squeezing roller 14, which could enable currents to flow from the squeezing rollers 14 to the photosensitive belt 1.
The charging performance of the squeezing rollers 14 varies with a critical potential related to the townsend discharge occurrence, a time constant to be determined by characteristics of the squeezing rollers 14 and the photosensitive belt 1, and a charging time.
Meanwhile, in a conventional developing unit 10, since the squeezing rollers 14 are supported by one cylindrical backup roller 15 and contacted with the photosensitive belt 1, the width of the squeeze nip formed between the photosensitive belt 1 and the squeezing roller 14 is short. The shortness of the squeeze nip means that time required to charge the photosensitive belt 1 by the squeezing rollers 14 is short. Accordingly, in the conventional developing unit 10, since the charging time for the photosensitive belt 1 is so short that the photosensitive belt 1 is not charged with an even potential in order for an electrostatic latent image to be formed for different colors, there exists a problem in that a printing quality is deteriorated.
Further, in the conventional developing unit 10, since a resistance of the contact layer 14b of each of the squeezing rollers 14 varies with changing temperature, a critical potential for the townsend discharge occurrence varies. Accordingly, since a charging potential of the photosensitive belt 1 varies with temperature, there exists a problem in that an optimum image formation condition changes.
The present invention has been devised to solve the above problems of the related art, and accordingly, it is an object of the present invention to provide a squeezing device and a developing unit employing the same, capable of securing charging time sufficiently.
Another object of the present invention is to provide a squeezing device that is stable with temperature changes and a developing unit employing the same.
The above object is accomplished by a squeezing device of a wet printer for separating a liquid carrier element not used for forming an image from ink supplied to a photosensitive belt in accordance with the present invention, including a squeezing roller mounted to be contacted with the photosensitive belt, a bias applying unit for biasing the squeezing roller at a certain potential, and a plurality of backup rollers mounted side by side to be opposite to the squeezing roller across the photosensitive belt, which is placed between the plurality of backup rollers and the squeezing roller.
The squeezing roller includes a core to which a bias potential is applied to charge the photosensitive belt by the bias applying unit, and a contact layer surrounding an outer circumference of the core, and preferably further includes a heater mounted in the core for heating the contact layer.
The above object is also accomplished by a developing unit of a wet printer for developing an electrostatic latent image by supplying ink to a photosensitive belt in accordance with the present invention, including an ink supplying unit for supplying ink to the photosensitive belt, a squeezing roller mounted in contact with the photosensitive belt for separating liquid carrier element, which is not used for forming an image, from the ink supplied to the photosensitive belt, a bias applying unit for biasing the squeezing roller at a certain potential, and a heater for heating the squeezing roller.
Another object is accomplished by a squeezing device having a squeezing roller mounted to be contacted with the photosensitive belt, a bias applying unit for biasing the squeezing roller at a certain potential, and a heater for heating a surface of the squeezing roller at a certain temperature.
Another object of the present invention is also accomplished by a developing unit of a wet printer for developing an electrostatic latent image by supplying ink to the photosensitive belt in accordance with the present invention, including an ink supplying unit for supplying ink to the photosensitive belt, a squeezing roller mounted in contact with the photosensitive belt for separating liquid carrier element, which is not used for forming an image, from the ink supplied to the photosensitive belt, a bias applying unit for biasing the squeezing roller at a certain potential, and a plurality of backup rollers mounted side by side to be opposite to the squeezing roller across the photosensitive belt, which is placed between the plurality of backup rollers and the squeezing roller.
It is preferable that the developing unit further includes a heater for heating the squeezing roller.